The present invention concerns shock damping and involves shock damper coatings intended to be applied in contact with an area to be protected.
Such coatings can have numerous applications. The following will be cited by way of example, without this list being in any way restrictive: the protection of objects in packaging, the protection of static or mobile installations against various kinds of projectiles (for example shrapnel, missiles, etc.), the protection of oil tankers, off-shore oil rigs and port terminals, against related objects, the protection of sensitive installations (nuclear power stations, hazardous plants) against crashing aircraft, and the protection of cable cars, etc.
Apart from protection against shocks in those types of applications just indicated, protective coatings according to the invention may likewise find applications in protection against explosions. However, as will appear later, the structure of the coating according to the invention has been developed to enable the crushing area of the coating to be widened with respect to the impact zone the result of this is that this coating will be useful where the shockwave caused by the explosion produces a localized impact. It may thus be a matter of protecting a ship's hull against the effect of mines or charges which act on contact with the hull.
When two bodies moving in relation to each other collide, the shock produces stresses at the point of impact, and these stresses deform the two moving bodies. These deformations are more or less severe, depending on the toughness of the body under consideration; moreover, for the smallest stresses (below the elastic limit, which varies according to the body, and may even vary according to the velocity of the impact), the deformation is reversible: the energy of the impact is absorbed by the deformation, then restored by repelling the "aggressor" moving body. Above the elastic limit the body is "crushable" and the deformation is irreversible. The energy of the impact is thus absorbed in the form of work required to produce the deformation (crushing, internal displacement, fractures, etc.). This mechanism is currently used to protect a surface or a volume against impacts with the aid of a layer of damping material.
Damping materials therefore exist, such as foams made of plastic material, glass or others, or products that are highly porous, such as stacks of hollow beads which absorb energy in proportion to the crushed volume, as illustrated in FIGS. 1a and 1b. FIG. 1a shows a moving body 1 at the instant of its falling onto a damping material 2 or crushing material covering a surface 3 to be protected. The moving body 1 penetrates the crushing material 2, and goes deep of a volume W of the crushing material 2, which slows it down (FIG. 1b). The energy absorbed corresponds to that required to compress this volume W of the crushing material 2 into a volume W/a, a being the crush coefficient. This crushed volume W/a is made to move ahead of the moving body 1, increasing as the latter advances until it stops, if the thickness of the crushing material 2 is sufficient, that is to say when the energy used up by compression is equal to the initial energy of the moving body 1. Otherwise, the surface to be protected 3 itself has to absorb the residual energy.
In the case of a material 2 whose crushing stress is Y.sub.e, the advance X of the moving body 1 of mass M and velocity V, exerting the impact on a surface area S is approximately ##EQU1##
The crushing material 2 still not crushed transmits stresses to the surface to be protected 3, which it should be able to accept.
The thickness required to absorb the kinetic energy of the moving body (1/2 MV.sup.2) is inversely proportional to the surface area S (see formula I).
In the mechanism just explained with reference to FIGS. 1a and 1b, the cross section of material which is affected by the compression phenomenon is more or less equal to the cross section S of the moving body 1. It is obviously preferable to cover the crushing material 2 by a skin 4, deformable but more rigid than the material, as is shown in FIG. 2a. FIGS. 2a, 2b and 2c illustrate the successive penetration of the moving body 1 into a crushing material 2 covered by skin 4. As can be seen in FIG. 2b, by accepting a certain deformation, the skin 4 widens the surface area of the absorbent zone. However, in the majority of cases this widening is extremely limited. In fact this skin 4 must have sufficient rigidity in order to crush the underlying material 2, but then the action of the moving body 1 on this rigid skin 4 creates, around the perimeter of the impact zone, a significant stress which produces tearing of the skin 4: the moving body 1 acts as a punch and the crushed zone is very slightly enlarged (FIG. 2c). It must therefore be very thick in order to absorb the incident energy.
The object of the present invention is to propose a damper coating, the structure of which is designed to dampen the impact under a reduced thickness.